In the field of underground construction of seepage barriers, clay/cement suspensions are used either by injecting them into the ground through grout holes or applying them as a slurry between the walls of vertical trenches and allowing them to set in place so as to form relatively impervious barriers. These techniques are used to contain groundwater when an impoundment is created or to contain contaminated groundwater or leachates of industrial and sanitary origins. Chronologically, stable slurries were first developed in the area of grouting before attention was directed to the application of slurries for trenched cutoff wall construction. Caron et al describe the latter method in U.S. Pat. No. 3,759,044 wherein panelized trench excavation is employed using clamshell equipment. In today's state of the art, hydraulic excavators are used for trenching down to 70 feet and a combination of both a hydraulic excavator and clamshell are used to construct deeper trenches built in a continuous manner and without distinct panelization.
Heretofore, the clay/cement slurries employed in the construction of these barriers have been aqueous slurries of bentonite and Portland cement. Bentonite must be hydrated with clean fresh water prior to the introduction of the cement which establishes a first constraint since the job site must have available the proper quality of water. Further, any contact with even minute quantities of Portland cement will contaminate the bentonite during its hydration and affect the characteristics of the bentonite slurry in a detrimental manner (floculation and filtrate loss). For this reason, bentonite and cement as dry materials must be stored carefully and away from each other and separate mixers are generally used for the hydration of the bentonite and for the subsequent addition of cement. Chemical additives are often used in an attempt to mitigate against these adverse factors but always at a premium.
Since slurry trenched cutoff walls do not require structural strength, the main characteristic of the end product is its permeability. Tested under standard procedures, such cutoff wall permeabilities range between 10.sup.-5 and 10.sup.-6 cm/sec. A second characteristic which bears on economics as well as on the properties of the final product, is the filtrate loss or the ability of the cement bentonite slurry to release water by filtration through the walls of the trench until the set has taken place. Depending on the cement/bentonite formulation and the chemical additives used, the A.P.I. standard test will give a filtrate of 130 to 250 cc. This is to be compared with the filtrate loss of pure bentonite slurry which will vary between 12.5 to 30 cc.
The rheology of the cement/bentonite system must be such that in a fresh state the slurry permits the performance of the trench excavation in good conditions by remaining stable, having a viscosity such that solids removed from the ground and not spoiled by the trenching equipment will remain dispersed in suspension, and not setting before a particular section of trench is fully excavated. These characteristics generally require minute quantities of chemical additives either organic or mineral but their cost is significant.
In the United States, the bentonite used is a sodium montmorillonite originating from Wyoming or South Dakota and the Portland cement is a Portland cement type I, II or III and eventually type IV. Although these materials can be used for impounding uncontaminated groundwater, care must be given when the intended application is the impounding of waste substances. Montmorillonite, in its hydrated form, has a plate structure where possible ion exchanges can result in the collapse of the plate structure thus causing the shrinkage of the clay. An increase in permeability can result. Its use is more a matter of derivation of technology inherited from the oil drilling industry and a matter of convenience, given its commercial availability, than the resulting of a scientific selection.
Portland cement in bentonite/cement slurries satisfies most normal uses in natural groundwater seepage control and also many cases of waste containment, particularly hydrocarbons. However, very low pH mediums can be detrimental to the cement in a cement/bentonite composition although to a lesser degree than would be in the case of a concrete. Free lime may become a source of increased permeability of the cement/bentonite if allowed to leach out over the long term. The problems with Portland cement are also of a practical nature: the phenomenon of false set creates a sudden rise in viscosity that must be countered with energetic mixing until the mix returns to an acceptable viscosity. When lower permeabilities must be achieved, a high proportion of cement must be used with the eventual addition of mineral fillers, thus creating serious difficulties in maintaining a workable viscosity and sufficient time for trenching before set. In the latter case, both fluidifiers and retarders are used systematically. Consequently, the cost for materials for achieving a permeability in the range of 10.sup.-6 to 10.sup.-7 cm/sec can be double the one required for the range between 10.sup.-5 and 10.sup.-6 cm/sec.
U.S. regulatory agencies and engineering bodies involved with environmental protection have determined and established as a minimum performance criterion of imperviousness for clay liners and barriers (which includes all forms of slurry trenched cutoffs) a coefficient of 10.sup.-7 cm/sec. Therefore, as discussed above, the performance of cement/bentonite slurry trenches is generally substandard and restricts significantly the possible use of self-hardening cement/bentonite slurries for toxic and hazardous waste containment applications. Needless to say, a real need exists for a clay/cement composition which remedies this situation and provides construction simplications that results in considerable savings.